1. Field of the Invention
The present invention relates to an explosion-proof hermetically sealed cell comprising a liquid active material. In particular, the present invention relates to a hermetically sealed cell comprising a liquid active material, which cell has improved actuation-reliability of an explosion-proof mechanism.
2. Description of the Related Art
In a so-called liquid active material cell comprising an oxyhalide compound which is in a liquid state at room temperature (e.g., thionyl chloride, sulfuryl chloride, phosphoryl chloride, etc.) as a positive active material and a negative electrode which comprises an alkali metal (e.g., lithium, sodium and potassium) and their alloy, since the positive active material and the negative electrode material are both reactive with water, a hermetic seal is used to close an opening of a cell container for containing those materials to increase sealing tightness.
The cell utilizing the hermetic seal has an advantage that it is tightly closed and has a good shelf life. But, when the cell is exposed to abnormal conditions, such as a high temperature or charging under a high voltage, the internal pressure of the cell abnormally increases and, in an extreme case, the cell bursts with a loud noise because of the tight seal, so that the contents of the cell are scattered around and will contaminate an the equipment using the cell.
To avoid bursting of the cell, an explosion-proof mechanism is provided in the cell. For example, as shown in FIG. 1, a thin-wall part 4 in the form of a cross is provided by grooves at the bottom 2 of cell 1. A center part of the bottom protrudes as 2a. The cell comprises a negative electrode 11, a separator 12, a positive electrode 13, an electrolyte liquid 14, a positive current collector 15, and a cell lid 16. The cell lid 16 comprises a body 17, an insulating layer 18 and a positive terminal 19. The cell further comprises a bottom insulating material 20, an upper insulating material 21 and a resin layer 22.
However, the cell is not necessarily used under expected conditions, but it may be exposed to unexpected conditions even when it is installed in equipment which uses the cell. Therefore, the cell should be designed so that it is safe under such unexpected conditions.
After shipping, a user installs the cell on a printed circuit board. In such a case, a free end of an external lead wire, which is connected to the cell by, for example, laser beam spot welding before shipping, is dipped in a solder bath containing molten solder, and then the cell is connected to the printed circuit board using the solder. In the soldering step, even if the cell is accidentally dropped in the solder bath, and a large amount of heat is applied to the cell so that the cell is very quickly heated, the cell should be safe by the actuation of the explosion-proof mechanism.
To confirm the safety of the cell when the cell is dropped in the solder bath accidentally, the cell was intentionally dropped in the solder bath, and it was found that, the thin-walled part was broken by the increase of internal pressure, and a part of lithium was made molten since the temperature rose quickly, so that the molten lithium flowed out from the broken thin-walled part together with the electrolyte liquid. Then, the molten lithium was ignited, and in turn the cell burst due to generated high pressure.
To secure the reliable actuation of the explosion-proof mechanism, it may be contemplated to reduce the thickness of the thin-wall part of the cell bottom so that the thin-wall part is completely broken before the lithium becomes molten. However, when the thickness of the thin-wall part is reduced to about 0.04 .mu.m, a mold for press molding the thin-wall part becomes heavily worn, and the molding cannot be carried out stably for a long time.
Further, when the lead wire is connected to the cell, the lead wire having a ring-form terminal is bonded to the bottom of the cell container by spot welding. In this welding step, if a position of the ring-form terminal shifts so that the cell container is directly heated by the welding heat, the lithium negative electrode, which is press bonded to the container, is made molten, and a cell reaction proceeds at once, so that the cell may burst by the volume expansion due to heat generation.